High-performance broadband flexible photodetector based on Gd3Fe5O12-assisted double van der Waals heterojunctions

Flexible photodetectors are fundamental components for developing wearable systems, which can be widely used for medical detection, environmental monitoring and flexible imaging. However, compared with 3D materials, low-dimensional materials have degraded performance, a key challenge for current flexible photodetectors. Here, a high-performance broadband photodetector has been proposed and fabricated. By combining the high mobility of graphene (Gr) with the strong light–matter interactions of single-walled carbon nanotubes (SWCNTs) and molybdenum disulfide (MoS2), the flexible photodetector exhibits a greatly improved photoresponse covering the visible to near-infrared range. Additionally, a thin layer of gadolinium iron garnet (Gd3Fe5O12, GdlG) film is introduced to improve the interface of the double van der Waals heterojunctions to reduce the dark current. The SWCNT/GdIG/Gr/GdIG/MoS2 flexible photodetector exhibits a high photoresponsivity of 47.375 A/W and a high detectivity of 1.952 × 1012 Jones at 450 nm, a high photoresponsivity of 109.311 A/W and a high detectivity of 4.504 × 1012 Jones at 1080 nm, and good mechanical stability at room temperature. This work demonstrates the good capacity of GdIG-assisted double van der Waals heterojunctions on flexible substrates and provides a new solution for constructing high-performance flexible photodetectors.


Introduction
With the development of wearable devices, the requirements for photodetectors are developing in the direction of being portable and flexible 1,2 . Photodetectors are widely used in various fields, such as medical detection 3,4 , environmental monitoring 5,6 , and imaging 7,8 . Recently, advances in flexible photodetectors have been accelerated by the emergence of low-dimensional materials 9 , such as the carbon family 0D material C 60 10 , 1D material carbon nanotube 11 , 2D material graphene 12 , the transition metal sulfide family [13][14][15] , black phosphorus 16 , hexagonal boron nitride and other graphene-like 2D materials [17][18][19] . The unique optical, electrical, and mechanical properties of low-dimensional materials make them important for the fabrication of high-performance flexible photodetectors.
However, low-dimensional materials have weak light absorption, which has limited the photoresponsivity of flexible photodetectors 20,21 . There is a trend of combining two-dimensional materials with different properties through van der Waals heterojunctions to improve flexible photodetectors 22 . Li et al. reported a laser-reduced graphene oxide/CsPbBr 3 van der Waals heterostructure flexible photodetector with a maximum photoresponsivity of 135 mA/W, a minimum NEP of 0.002 nW/Hz 1/2 and a maximum specific detectivity of 1.6 × 10 11 Jones 23 . Combining graphene with transition metal sulfides 24,25 combines the high mobility of graphene with the nonzero band gap of transition metal sulfides to obtain flexible photodetectors with high performance in the visible light range 26 . However, due to the large surface-to-volume ratio of two-dimensional materials, they are easily affected by water vapor and impurities when exposed to air, so such devices are unstable [27][28][29] . In addition, most flexible photodetectors constructed of low-dimensional materials are based on the principle of the photovoltaic effect, resulting in a large dark current, which affects the ability of the device to detect weak signals 30 .
Double van der Waals heterojunctions are constructed from several low-dimensional materials through weak van der Waals forces and do not need material doping 31,32 . Moreover, the asymmetry of the two heterojunctions increases the built-in electric field, accelerating the separation of carriers and enhancing light absorption, which greatly improves the responsivity of the device 33,34 . Through the combination of different materials, the response spectral range of the device is broadened. However, the large built-in electric field leads to a large majority of carrier dark current. As a transparent insulating ferromagnetic material, gadolinium iron garnet (Gd 3 Fe 5 O 12 , GdIG) has a high dielectric constant and insulating properties 35 . Therefore, it can be used as an intercalation layer to improve the interfacial barrier between materials, block carrier transport, and reduce dark current 36 . The thickness and uniformity of the GdIG film can be controlled by magnetron sputtering technology to achieve a barrier to water vapor and impurities and improve the stability of the device.
In this work, we developed a GdIG intercalationassisted double van der Waals heterojunction flexible photodetector operating in the visible to near-infrared (NIR) region. The double van der Waals heterojunctions consist of single-walled carbon nanotube film/graphene/ molybdenum disulfide film (SWCNT/Gr/MoS 2 ). The MoS 2 and SWCNTs enhance the broad light spectrum absorption of the double heterojunctions and thus lead to a prominent photocurrent upon illumination with visible and NIR light. Moreover, under the action of a large builtin electric field generated by double heterostructures, the SWCNT/Gr/MoS 2 flexible photodetector exhibits a responsivity of 17.009 A/W and a specific detectivity of 2.258 × 10 10 Jones at 450 nm. Additionally, the GdIG films, which had excellent uniformity and continuity, acted as intermediate layers to optimize the doubleheterojunction interface, increasing the barrier height between the heterojunctions and blocking the diffusion dark current. The SWCNT/GdIG/Gr/GdIG/MoS 2 flexible photodetector exhibits room-temperature weak-light detection with a responsivity of 109.311 A/W and a specific detectivity of 4.504 × 10 12 Jones at 1080 nm. Compared to the Gr/MoS 2 flexible photodetector, the device has nearly 30 times higher responsivity and three orders of magnitude higher specific detectivity. Moreover, the device exhibits good mechanical stability, as detected by bending tests under different curvature radii. The investigations provide a new design concept to fabricate highperformance broadband photodetectors on flexible substrates, which accommodates various applications for wearable consumer electronics.

Device fabrication
First, a clean PET substrate was prepared, which was cleaned with acetone, ethanol and deionized water; then a square window of 1 × 1 mm was defined on the Al surface by UV lithography and electron beam deposition technology. Then, the MoS 2 dispersion was uniformly spincoated onto the surface of Al by a spin processor and dried by a drying table. Next, the GdIG film was deposited on the MoS 2 window, forming an interfacial layer, and the Al layer was removed by the Al etching solution to obtain GdIG and MoS 2 films with a size of 1 × 1 mm. Then, the prepared graphene film was used to cover the surface of the GdIG film and etched with oxygen plasma to obtain a graphene film with a size of 4 × 1 mm. A window of 1 × 4 mm was defined on the Al surface by UV lithography and electron beam deposition technology. The GdIG film was deposited on the Al window again, forming an interfacial layer, and SWCNT and GdIG films with a size of 1 × 4 mm were prepared by the same steps as the MoS 2 film. Finally, 20 nm Ti and 80 nm Au were deposited on both ends of the graphene and SWCNT films using UV lithography and electron beam evaporation techniques again as source-drain electrodes ( Supplementary Fig. S1). The detailed synthesis/processes of graphene, MoS 2 , SWCNT and GdIG film are given in the supporting information.

Device characterization and measurements
Scanning electron microscopy (SEM) images were obtained using a Carl Zeiss Gemini SEM 500 at an acceleration voltage of 5 kV. Atomic force microscope (AFM) images were obtained using INNOVA with a scan line rate of 256. Transmission electron microscopy (TEM) samples were prepared using a Gatan691 ion gun. The TEM images were obtained using a JEOL JEM-2100 operated at 300 kV in bright-field TEM mode and high-resolution TEM mode. Raman spectroscopy was measured using a confocal Raman microscope (Horiba JOBIN YVON, hr800) with an excitation wavelength of 532 nm at an output power of 100 mW. X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB Xi + )) was used to determine the functional groups and required binding energies of the thin films. The electronic measurements were conducted using a semiconductor analyzer (Keysight B1500A). For optoelectrical testing, the light was illuminated from the top side of the device. Visible and NIR light were provided by a Xe lamp with a color filter (~300-1500 nm). The power of light sources was measured by commercial Si (Thorlabs S120VC) photodetectors.

Results and discussion
Design of SWCNT/Gr/MoS 2 double heterojunction flexible photodetector Fig. 1a shows a schematic illustration of a flexible photodetector with SWCNT/Gr/MoS 2 double heterostructures. On a flexible polyethylene terephthalate (PET) substrate, the MoS 2 film is first used as a visiblewavelength absorption layer 13 , above which is a graphene layer. Gr not only forms a van der Waals heterojunction with MoS 2 and the SWCNT film but also transports carriers as a transparent electrode. The SWCNT film is used as a near-infrared-wavelength absorption layer on the top layer 11 and forms a crisscross pattern with Gr. Finally, Au/Ti electrodes are evaporated on both ends of the graphene and SWCNT films.
The principle of the flexible photodetector based on the double heterojunctions of SWCNT/Gr/MoS 2 is shown in Fig. 1b. The inset shows the double heterojunctions under equilibrium conditions, in which the SWCNT film forms a heterojunction with graphene to absorb the NIR spectrum and produces a hole accumulation layer and is also used as a transparent electrode to transport hole carriers; Gr and MoS 2 form a heterojunction to absorb the visible spectrum by MoS 2 ; and Gr is also used as a transparent electrode to transport electron carriers ( Supplementary  Fig. S2). Based on the van der Waals heterojunctions and double heterojunctions, the SWCNT/Gr/MoS 2 flexible photodetector utilizes the high absorption characteristics of SWCNTs in NIR light, the high absorption of MoS 2 film in visible light, the high mobility of graphene, and the large built-in electric field of double heterojunctions to effectively separate the photogenerated carriers generated by SWCNTs and MoS 2 so that the spectral response of the double heterojunction photodetector ranges from visible light to the infrared band, enhancing the performance of the flexible photodetectors. Fig. 1c is a photograph of the SWCNT/Gr/MoS 2 flexible photodetector after fabrication. Fig. 1d shows a schematic diagram of the device fabrication process. the SWCNT film is completely and uniformly stacked on the graphene, forming a dense heterojunction, and the MoS 2 film cannot be seen. Additionally, cross-sectional transmission electron microscopy (TEM) of the SWCNT/ Gr/MoS 2 double heterojunctions further indicated the high quality of the transferred heterostructures. The cross-section of MoS 2 , graphene and SWCNT can be clearly seen in Fig. 2c, and the figure also shows excellent coordination between them.
To further verify that the compositions of the device contain heterojunctions, unlike the original materials, the Raman spectra of the individual materials were compared with the Raman spectrum of the photodetector. As shown in Fig. 2d, two peaks of the in-plane (E 2g ) and out-of-plane (A 1g ) modes are shifted for MoS 2 . In contrast to the original MoS 2 , the E 2g peak in MoS 2 /Gr/SWCNT was downshifted from 384 to 379 cm −1 and the A 1g peak upshifted from 403 cm −1 to 404 cm −1 , which indicated photogenerated carrier transfer between heterojunctions and is similar to the reported data on photogenerated charge transfer between MoS 2 and Gr 21 . Fig. 2e shows the defect mode peak (D), vibration mode peak (G) and frequency-doubling mode peak (2D) of Gr and SWCNTs. After the heterojunction is formed, the D peak is very weak, and the intensities of both the G peak and the 2D peak increase, confirming that Gr and SWCNT maintain The interaction between the double heterojunctions is crucial for the transfer of photogenerated carriers and thus the photoresponsivity performance of the SWCNT/ Gr/MoS 2 flexible photodetector. As cross-validation, band theory was further employed to analyze the photoresponse mechanism of the photodetector 37 . When MoS 2 , Gr and SWCNT are in contact with each other, due to the difference in their work functions, electrons in MoS 2 flow to Gr, and holes in SWCNT flow to Gr, forming double built-in electric fields at the interface. The built-in electric field direction of MoS 2 points from Gr and from Gr to SWCNT, forming a larger potential barrier (qV D1 + qV D2 ) at the interface than the Gr/MoS 2 heterojunction. Under the action of the built-in electric field, the mobile carriers move in the opposite direction: electrons in the SWCNT enter the MoS 2 through Gr, and holes in the MoS 2 enter the SWCNT. The potential energy generated by the electric fields leads to band bending until their Fermi levels are aligned to E f (SWCNT-Gr-MoS 2 ) and the carrier motion reaches equilibrium, as shown in Fig. 3a.
When the double heterojunctions are illuminated, photogenerated carriers are generated and separated by the built-in electric field. At this time, under the action of the built-in electric field, the photogenerated electrons generated by SWCNT are injected into the conduction band of graphene, and the hole carriers of higher concentration are injected into the SWCNT; meanwhile, the photogenerated holes generated by MoS 2 are injected into the valence band of graphene, and the electron carriers of higher concentration are injected into the MoS 2 , which in turn generates a photoelectromotive force. The larger built-in electric field generates a larger photoelectromotive force 32 , which lowers the potential barrier by qV and generates a larger photocurrent in the external circuit. With the injection of photogenerated holes, the Fermi level of SWCNT decreases from the initial offset height to E´f(SWCNT); with the injection of photogenerated electrons, the Fermi level of MoS 2 decreases from the initial offset height to E´f(MoS 2 ), up to the Fermi level of E´f(SWCNT) aligned. The photocurrent is saturated at this time, as shown in Fig. 3b.
It is now understood that double heterojunctions play an important role in modifying the built-in electric field, which is beneficial for high-performance photodetection. Therefore, the optical response of the SWCNT/Gr/MoS 2 flexible photodetector was measured and compared to that of the conventional construction. Fig. 3c illustrates the I-V curves at room temperature under darkness and 450 nm illumination with a power density of 20 mW/cm 2 . It can be seen from the figure that the device demonstrated negative photoconductivity (NPC), that is, the current of the device decreases with illumination. Because Gr is disturbed by impurities such as water vapor during the transfer process, it exhibits a p-doping effect at zero gate bias and is dominated by holes in the process of transferring current ( Supplementary Fig. S3). When light illuminates the device, because Gr is more conductive than MoS 2 , the carrier transport in the Gr/MoS 2 heterojunction is dominated by Gr. When the electrons in Mos 2 are injected into graphene, they compensate for the holes in p-doped graphene and reduce the carrier current of graphene, leading to the appearance of NPC. Photoresponsivity (R) and specific detectivity (D*) are important parameters to evaluate the photoresponse of detectors. Here, the responsivity R is defined as: I p refers to the photocurrent, P is the optical power density, and A is the effective photosensitive area of the device. I p is calculated as follows: I L is the total current under illumination, I D is the dark current, the D* is defined as: q is the charge of electrons. At a drain voltage of 3 V, the I p of the Gr/MoS 2 flexible photodetector is observed to be 0.323 mA, while the I p of the SWCNT/Gr/MoS 2 flexible photodetector is 3.402 mA. The photodetector exhibits an R of 17.009 A/W and a D* of 2.258 × 10 10 Jones at a 3 V drain voltage, but the dark current increases from 2.744 mA to 17.708 mA. In addition, the response time, defined as an increase in the photocurrent from 10 to 90%, is observed to be 498 ms, while the recovery time, defined analogously, is 290 ms, as shown in Fig. 3d. Finally, we compared the Gr/MoS 2 ( Supplementary  Fig. S4) and SWCNT/Gr/MoS 2 ( Supplementary Fig. S5) flexible photodetectors in the visible spectral range. It can be seen from Fig. 3e that compared with the Gr/MoS 2 flexible photodetector, the R of the SWCNT/Gr/MoS 2 flexible photodetector has been increased by nearly 10 times, and the spectral response range has also been broadened, which verifies that the double heterojunctions improve the responsivity and spectral response range. SWCNT/Gr/MoS 2 flexible photodetectors also exhibit excellent mechanical flexibility due to the use of twodimensional materials (Supplementary Fig. S6). However, due to the large built-in electric field of the double heterojunctions, the SWCNT/Gr/MoS 2 flexible photodetector has a large diffusion dark current, and the specific detectivity of the device is only 10 10 (Supplementary Fig. S7). Therefore, we considered suppressing the dark current of the device by inserting a thin insulating oxide layer at the interface of the heterojunction to improve the ability of the device to detect weak signals.

SWCNT/GdIG/Gr/GdIG/MoS2 flexible photodetector
To further enhance the performance of the double heterostructures, we increased the intercalation layer method to block the dark current. A cross-section illustration of the SWCNT/GdIG/Gr/GdIG/MoS 2 flexible photodetector is shown in Fig. 4a. As a transparent insulating ferromagnetic material, GdIG has a high dielectric constant and insulating properties. When the GdIG film is intercalated as the interlayer, SWCNTs, Gr and MoS 2 are spatially separated. To verify the effect of GdIG intercalation in reducing the dark current, we then compared the optical response curves of double heterojunction flexible photodetectors with and without GdIG intercalation, as shown in Fig. 4b. It can be seen from the figure that the I D of the SWCNT/GdIG/Gr/GdIG/MoS 2 flexible photodetector is nearly 10 times lower than that of the device without GdIG intercalation. Fig. 4c shows the responsivity and the specific detectivity of the SWCNT/GdIG/Gr/GdIG/MoS 2 flexible photodetector from the visible to NIR region. This device exhibits an R of 47.375 A/W and D* of 1.952 × 10 12 Jones at 450 nm; R of 109.311 A/W and D* of 4.504 × 10 12 Jones under 1080 nm at room temperature, which is comparable to the performance of the current commercial infrared InGaAs photodetectors (∼10 12 Jones) that require cryogenic cooling (4.2 K) 33,34 . Compared with the Gr/MoS 2 flexible photodetector, the R of the device is improved by nearly 30 times, and the D* of the device is improved by three orders of magnitude. This indicates that the GdIG films as intercalation layers bring about a better photo response capability for the detector.
For cross-validation, band theory was further employed to analyze the photoresponse mechanism of the SWCNT/ GdIG/Gr/GdIG /MoS 2 flexible photodetector. When the GdIG film is intercalated as an interlayer, SWCNTs and MoS 2 generate a potential along the GdIG film, which significantly modifies the heterojunction barrier and builtin electric field, as shown in Fig. 4d. Due to insufficient energy, under dark conditions, the thermally generated carriers are blocked from passing through the potential barrier so that the carrier dark current generated by the built-in electric field is suppressed (Supplementary Fig.  S8). In addition, it can be seen from Fig. 4e that when illuminated, the larger built-in electric field accelerates the photogenerated carriers, the 2 nm thin GdIG layer becomes ineffective, and photogenerated carriers are injected into the SWCNT and MoS 2 by the tunneling effect, resulting in a device with a higher photocurrent.
Furthermore, the practical suitability of the photodetector for high-performance weak-light photodetection was studied. We tested the broadband photoresponse capability of the device between 400-1500 nm using light with a microwatt-level power density, as shown in Fig. 5a. This demonstrated the broad spectral detection capability of the device under weak light intensities. We also analyzed the optoelectronic performance of the detectors at different optical power densities and weak drain voltages. Fig. 5b shows the photocurrent of the device with a drain voltage of 0.1 V under different optical power densities (3,9,33,50, and 410 μW cm −2 ) at a wavelength of 780 nm. The response time was observed to be 498 ms, while the recovery time was 628 ms, as shown in Fig. 5c. Through the above tests, it was shown that the I L and R of the device under illumination increased with increasing optical power density and bias voltage, and the device had good weak-light detection capability.
The stable photoresponse is crucial for flexible photodetectors under deformation. Artificial bending is a common method to test the mechanical flexibility of devices. Fig. 5d shows the impulse response characteristics of the device under different radii (R = 8, 7, 6, 5 mm, r is the bending radius) of artificial bending. It is obvious that the photocurrent remains almost unchanged after bending. In addition, the response time of the device remains unchanged in the bent state. The stable performance output at different bending levels ensures the practical application of our flexible photodetector in wearable devices.
The double-heterojunction flexible photodetector with a GdIG interlayer shows a high-performance photoresponse, including a specific detectivity > 10 12 , broadband absorption spectrum, high responsivity of 100 A/W, and good stability. To further verify the excellent performance of the photodetector, we compared our device with previously reported flexible photodetectors with different substrates, structures and 2D materials. Our device exhibits the advantages of high photoresponsivity and specific detectivity compared with other flexible photodetectors at room temperature, as shown in Fig. 6a, and the sensitive spectrum remains at the upper-middle level, as shown in Fig. 6b. These excellent properties validate that the SWCNT/GdIG/Gr/GdIG/MoS 2 double heterojunctions can serve as a high-quality flexible photodetector and may dominate next-generation wearable applications.

Conclusion
In summary, we fabricated a GdIG intercalation-assisted flexible photodetector based on double van der Waals heterostructures. The double heterojunctions prepared on the flexible substrate are able to achieve high responsivity by enhancing the built-in electric field. Through the combination of SWCNTs, Gr and MoS 2 , the flexible photodetector can achieve light absorption in the visible to NIR region at room temperature. To reduce the dark current of the device, we improved the double-      This study demonstrates that the combination of double heterojunctions with GdIG films as interlayers on a PET flexible substrate has application potential with high responsivity, a broad spectrum, and weak-light detection, which provides a new idea for improving the performance of flexible devices and has advantages for preparing highperformance flexible photodetectors.